Light emitting diode (led) test apparatus and method of manufacture

ABSTRACT

Embodiments relate to functional test methods useful for fabricating products containing Light Emitting Diode (LED) structures. In particular, LED arrays are functionally tested by injecting current via a displacement current coupling device using a field plate comprising of an electrode and insulator placed in close proximity to the LED array. A controlled voltage waveform is then applied to the field plate electrode to excite the LED devices in parallel for high-throughput. A camera records the individual light emission resulting from the electrical excitation to yield a function test of a plurality of LED devices. Changing the voltage conditions can excite the LEDs at differing current density levels to functionally measure external quantum efficiency and other important device functional parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/449,554, filed Jan. 23, 2017, commonly assigned and incorporated byreference herein for all purposes.

FIELD OF INVENTION

The present invention relates to light emitting diode (LED) devices.More particularly, embodiments of the invention relate to techniques,including methods and apparatus to functionally test a Light EmittingDiode (LED) array structure during the manufacturing process. In anexample, the method is useful in general LED device functional testingand is particularly useful for functionally testing micro-LED (uLED)devices that can be as small as a few microns on a side. Micro-LEDs aregrown on a support substrate utilizing techniques such asMetallo-Organic Chemical Vapor Deposition (MOCVD) among others. Beforethe individual devices are used in their final lighting or displayapplication, it is desirable to test the LED devices to achieve one ormore of the following: yield evaluation, binning, devicerepair/correction and collecting data for use in manufacturing processfeedback/feedforward.

BACKGROUND OF INVENTION

Light emitting diodes (LEDs) have been used as a replacement technologyfor conventional light sources. For example, LEDs are found in signage,traffic signals, automotive tail lights, mobile electronics displays,and televisions. Various benefits of LEDs compared to traditionallighting sources may include increased efficiency, longer lifespan,variable emission spectra, and the ability to be integrated with variousform factors.

Although highly successful, improved techniques for manufacturing LEDsis highly desired.

SUMMARY

During the LED manufacturing process, LED structures are formed on asubstrate using mass-production processes like those utilized by thesemiconductor industry. Process steps such as cleaning, deposition,lithography, etching and metallization are used to form the basic LEDstructure. To achieve mass-production scale manufacturing and lowercost, numerous devices are simultaneously formed on a substrate usingthese processes. Different substrates and materials are used dependingon the type of LED desired. For example, UV-emitting LEDs are typicallymade from Gallium Nitride (GaN) material that have usually been either aheteroepitaxial layer on sapphire or free-standing GaN substrates madeusing Hydride Vapor Phase Epitaxy (HVPE) or ammonothermal methods. Forother colors, GaAs or GaP substrates can be used. Recently, GaN, GaAs orother III-V semiconductor materials layer-transferred onto a supportsubstrate has become available as another starting substrate type. Otherpossible LED structures that can be tested using methods disclosed inthis invention are organic LED (OLED) device structures fabricated onplastic, glass or other suitable substrates.

Within the LED structure formation manufacturing process, variousoptical and other metrology tests are made to confirm quality andrepeatability. Once the LED structure formation has been completed, itis desirable to perform a functional test of each LED device before thedevice is mounted for use as a LED emitter within a package or as an LEDemitter within a display. Even if there is a common contact to alldevices (i.e. all cathodes are tied together), each individual anode ofeach device would still require individual contact in order tofunctionally test its opto-electronic characteristics. The device sizeand sheer volume of individual LED devices on a substrate makes this achallenging task. For example, a 6″ substrate with LED devices measuring250 μm on a side (typical of general lighting type LEDs) would containover 250,000 devices, each requiring a contact probe/measurement cycle.If the 6″ substrate contained micro-LED device structures of 20 μm on aside, there would be a need to contact each of the more than 40 milliondevices present on the substrate. There is therefore a need to developmethods that allows functional LED device testing without individualcontact.

Embodiments of the invention utilizes a non-direct electrical contactapproach where the current is injected through a capacitor formed usinga dielectric-coated field plate driven by a suitable voltage waveformsource. The other side of the dielectric surface forms a capacitorplaced in proximity to the plane of the individual LED contacts andspecific voltage waveforms are driven between the field plate electrodeand a common LED contact or a second capacitively-coupled LED contact.In a preferred embodiment, a voltage ramp drives the electrodes toforward bias the LEDs situated between these electrodes, developing adisplacement current that flows current into each of the large pluralityof LED devices in a parallel fashion. The functional response (lightemission) is then measured using an integrating camera disposed eitherabove the field plate or below the LED support substrate depending onthe embodiment. Image capture and processing can then extract manyfunctional device tests in parallel. In this manner, as few as twoelectrical contacts could functionally test LED devices numbering in themillions.

After each measurement, the capacitive field plate and other couplingcapacitance elements must be reset or in a manner that would not damagethe LED devices through excessive reverse bias voltage. A suitably slownegative voltage ramp would allow the LED device's minimum leakagecurrent to safely discharge the field plate capacitor without developingdamaging reverse bias conditions. Another measurement cycle can then berepeated.

Changing the forward bias drive voltage ramp would drive differingforward bias current density (A/cm²) into the LED devices, therebyallowing more complex functional test evaluations to be conducted.Device characterization data such as external quantum efficiency as afunction of forward bias current density made possible by selectingdifferent drive voltage waveforms are another feature of this invention.By modifying the field plate dielectric design and voltage ramp values,accurate current injection emission responses of a large plurality ofdevices can be detected over a large current density from about 0.001 to10 or more A/cm2.

One benefit afforded by this functional test method is the eliminationof high-pin count probe cards and probe needles that contact eachaddressable LED device under test. When such probe cards are used, eachLED device is contacted using one or more needle probe pins thatachieves electrical contact on a contact area by pressure and lateralmotion of the sharp metallic pin. This process almost always producecontact pad scratches that can lower LED device yield and reliability.Test reliability, fabrication and maintenance costs of using probe pincards having hundreds or even thousands of probe pins are also ofconcern. Elimination of probe card scratches or marks, increasing LEDdevice yield and reliability and avoiding the use of expensive andfailure-prone high-pin count probe cards is a key benefit of thisinvention.

Yet another benefit of afforded by this functional test method is theability to functionally test within the LED manufacturing process due tothe elimination of direct electrical contact. Clean-room compatible,scratch-free in-process testing of LED devices is a capability thatwould have otherwise being difficult or impractical to perform due tothe necessity of high-density particle generating pin cards use byconventional test methods.

Other benefits afforded by this functional test method is its generalapplicability to both small and large LED devices and scalability tolarge substrates. The field plate is a structure that appliescapacitance proportionally to area and thus, larger LED devices withmore area are excited with a larger effective capacitance while smallLED devices such as micro-LED devices are excited by a correspondinglysmaller capacitance. Large LEDs of millimeter size on a side down tomicro-LEDs as small as 10 μm or less on a side can therefore be testedwith few modifications of the apparatus. Substrate scalability usinglarger field plates or using a step/repeat method with a smaller fieldplate is practical and readily achievable. For the highest throughput,parallel processing of multiple cameras arranged in an array over alarge field plate would be able to functionally test all LED devices ona support substrate with as few as two electrical contacts and in someembodiments, with no direct electrical contact. Avoiding contacting eachindividual LED device that could number from many thousands to tens ofmillions on a substrate is a key benefit of this invention.

The method as described in this invention is described as CapacitiveCurrent Injection (C²I) functional testing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified cross-section of an LED structure.

FIG. 2 shows an LED support substrate containing LED device structureswithin an LED mass-production process.

FIGS. 3A-B show a top view (A) and a cross-sectional view (B) of a LEDsupport substrate with singulated LED devices isolated by streets.

FIG. 4 shows the LED support substrate with a non-singulated LED devicestructure with the top contact layer having sufficiently high sheetresistance to allow current injection functional test in the presence ofadjacent shorts.

FIG. 5A shows an embodiment of a field plate in close proximity to aportion of an LED device layer containing 4 LED devices on a supportsubstrate.

FIG. 5B shows the corresponding equivalent electrical circuit of theembodiment of FIG. 5A.

FIG. 6A shows the main Capacitive Current Injection (C²I) measurementsequence: Current Injection/Measurement (I), Hold (II), Discharge/Reset(III) phases.

FIG. 6B shows the corresponding LED current injected by the CapacitiveCurrent Injection (C²I) measurement sequence of FIG. 6A.

FIGS. 7A-B show two embodiments of a field plate with camera lookingthrough the field plate (A) and through the LED device support substrate(B).

FIG. 8 shows the expected current density (A/cm2) versus dV/dT voltageramp of a preferred embodiment.

FIG. 9 shows a substrate scale method to attach the field plate onto asupport substrate containing LED device structures using vacuumdeveloped in the space between the field plate and the supportsubstrate.

FIG. 10 shows a smaller field plate & camera optical system in astep/repeat mechanical configuration.

FIG. 11 shows a circuit model used to simulate the C²I functional testmethod according to an embodiment.

FIGS. 12A-D show the detailed sequence of the currentinjection/measurement phase I of an embodiment.

FIGS. 13A-D show the detailed sequence of the currentinjection/measurement phase III of an embodiment.

FIGS. 14 A-D show a longer time axis (200 msec) showing 4 measurementsequences of an embodiment.

FIG. 15A shows an embodiment of a field plate in close proximity to aportion of a LED device layer containing 4 LED devices on a supportsubstrate with a buried common contact and optional dielectric layer(i.e., “leaky” dielectric layer) and coupling gap medium to allow DCbias functional test.

FIG. 15B shows the corresponding equivalent electrical circuit of theembodiment of FIG. 15A.

FIG. 16 shows an embodiment of a field plate without a dielectric inclose proximity to a portion of a LED device layer containing 4 LEDdevices on a support substrate with a buried common contact. Theconfiguration using an external load resistor and coupling capacitor andDI water gap medium allows DC bias and AC pulsed functional test.

FIG. 17A shows an embodiment of a field plate in close proximity to aportion of a LED device layer containing 4 LED devices on a supportsubstrate with a buried common contact and dielectric layer.

FIG. 17B shows the corresponding equivalent electrical circuit of theembodiment of FIG. 17A.

FIG. 18 shows an embodiment of a field plate in close proximity to aportion of a LED device layer containing 4 LED devices on a supportsubstrate that is used as a dielectric layer for capacitively couplingthe second electrode.

FIG. 19 shows three LED structures A, B and C, in which structure A is avertical LED structure with top surface area accessing the top contactby top capacitive coupling and the bottom surface area accessing thebottom contact by bottom capacitive coupling, structure B is a lateralMESA-type LED structure with top surface area accessing both the topcontact and bottom contact by top capacitive coupling and the bottomsurface area accessing the bottom contact by bottom capacitive coupling,and structure C is a lateral MESA-type LED structure with a small topvia opening to the bottom contact. Except for differing relativecapacitance values, it is electrically similar to structure B.

FIG. 20 shows three different capacitive coupling configurations A, Band C, where configuration A uses a common top field plate electrode,configuration B uses a patterned top field plate electrode only couplingto the top (anode) contact, and configuration C uses a patterned topfield plate electrode with separate top (anode) and bottom (cathode)contacts.

FIG. 21 shows a histogram plot of several LED devices falling withinsmall ranges of Data_(n) values (called channels or bins) on thevertical scale as a function of Data_(n) in the horizontal scale.

FIG. 22 shows a use of C²I functional test method within a roll-rollcontinuous manufacturing process where sensors are moved synchronouslywith the LED structure/film. Multiple sensor assemblies are used toeliminate measurement gaps.

FIG. 23 shows a use of C²I functional test method within a roll-rollcontinuous manufacturing process where sensors are stationary and adischarge field plate assembly is used to safely discharge the LEDstructure.

FIG. 24A shows an embodiment of the discharge field plate assembly withspatially decreasing voltage as a means of safely discharging the LEDstructure after the measurement phase.

FIG. 24B shows an embodiment of the discharge field plate assembly withspatially increasing dielectric thickness as a means of safelydischarging the LED structure after the measurement phase.

DETAILED DESCRIPTION

A further explanation of LEDs is found throughout the presentspecification and more particularly below. In an example, one type ofLED is an organic light emitting diode (OLED) in which the emissivelayer of the diode is formed of an organic compound. One advantage ofOLEDs is the ability to print the organic emissive layer on flexiblesubstrates. OLEDs have been integrated into thin, flexible displays andare often used to make the displays for portable electronic devices suchas cell phones and digital cameras.

Another type of LED is a semiconductor-based LED in which the emissivelayer of the diode includes one or more semiconductor-based quantum welllayers sandwiched between thicker semiconductor-based cladding layers.Some advantages of semiconductor-based LEDs compared to OLEDs caninclude increased efficiency and longer lifespan. High luminousefficacy, expressed in lumens per watt (lm/W), is one of the mainadvantages of semiconductor-based LED lighting, allowing lower energy orpower usage compared to other light sources. Luminance (brightness) isthe amount of light emitted per unit area of the light source in a givendirection and is measured in candela per square meter (cd/m²) and isalso commonly referred to as a Nit (nt). Luminance increases withincreasing operating current, yet the luminous efficacy is dependent onthe current density (A/cm²), increasing initially as current densityincreases, reaching a maximum and then decreasing due to a phenomenonknown as “efficiency droop.” Many factors contribute to the luminousefficacy of an LED device, including the ability to internally generatephotons, known as internal quantum efficiency (IQE). Internal quantumefficiency is a function of the quality and structure of the LED device.External quantum efficiency (EQE) is defined as the number of photonsemitted divided by the number of electrons injected. EQE is a functionof IQE and the light extraction efficiency of the LED device. At lowoperating current density (also called injection current density, orforward current density) the IQE and EQE of an LED device initiallyincreases as operating current density is increased, then begins to tailoff as the operating current density is increased in the phenomenonknown as the efficiency droop. At low current density, the efficiency islow due to the strong effect of defects or other processes by whichelectrons and holes recombine without the generation of light, callednon-radiative recombination. As those defects become saturated radiativerecombination dominates and efficiency increases. An “efficiency droop”or gradual decrease in efficiency begins as the injection-currentdensity surpasses a low value, typically between 1.0 and 10 A/cm².

Semiconductor-based LEDs are commonly found in a variety ofapplications, including low-power LEDs used as indicators and signage,medium-power LEDs such as for light panels and automotive tail lights,and high-power LEDs such as for solid-state lighting and liquid crystaldisplay (LCD) backlighting. In one application, high-poweredsemiconductor-based LED lighting devices may commonly operate at400-1,500 mA, and may exhibit a luminance of greater than 1,000,000cd/m². High-powered semiconductor-based LED lighting devices typicallyoperate at current densities well to the right of peak efficiency on theefficiency curve characteristic of the LED device. Low-poweredsemiconductor-based LED indicator and signage applications often exhibita luminance of approximately 100 cd/m² at operating currents ofapproximately 20-100 mA. Low-powered semiconductor-based LED lightingdevices typically operate at current densities at or to the right of thepeak efficiency on the efficiency curve characteristic of the LEDdevice. To provide increased light emission, LED die sizes have beenincreased, with a 1 mm² die becoming a fairly common size. Larger LEDdie sizes can result in reduced current density, which in turn may allowfor use of higher currents from hundreds of mA to more than an ampere,thereby lessening the effect of the efficiency droop associated with theLED die at these higher currents.

LEDs have been used in portable devices such as watches, smartphones andlaptops as well as computer monitors and TV displays for many yearshowever only indirectly as an alternative white light source forLiquid-Crystal Display (LCD) display technologies. These were called“LED” TVs and the like, but the actual LEDs were predominantly GaN-basedwhite LEDs to illuminate the backlight in lieu of the cold fluorescentlamp (CFL) backlight sources used before. The color pixel generationcontinued to be based on LCD technology that worked by a lightsubtraction process where colors are generated by blocking other colorsusing an intervening color filter. For example, a red pixel would begenerated by a red color filter that blocked the green and blue portionof the backlight LED white spectrum. Grey scale (light intensity of thepixel) occurred by modulating light polarization through aliquid-crystal cell placed between two crossed polarizers along thelight path.

Although the LED backlight driven LCD display technology was moreefficient and reliable than the CFL backlit version, the technology isstill not power efficient. The reason is simple: although the LED whitebacklight devices can be fairly efficient in terms of external quantumefficiency (photons emitted per electrical carriers injected into theLED device), there are numerous inefficiencies in the rest of this LCDdisplay technology. The first polarizer will cut a little half of thenon-polarized white backlight, then each pixel is colorized bysubtracting ⅔ of the remaining light (R without GB for red, G without RBfor green and B without RG for blue). Other losses include pixel fillfactor and film/LCD cell absorption and scattering. The total lightoutput is therefore less than about ⅙ of the white LED backlightintensity.

The trend is for more power efficient and bright display technologies,especially with portable, battery operated devices where battery life isa key factor. Micro-LED is a promising technology for achieving higherpower efficiencies. In a micro-LED display, a small LED device placedwithin a pixel area is directly driven to generate light in a directemissive manner. Color can be generated either by (i) utilizing UV-LEDs(i.e. GaN-based) with color phosphors to generate the pixel colors byphoton down conversion and/or (ii) by using LEDs that generate the colordirectly (i.e. AlGaAs, GaAsP, AlGaInP, GaP for red, GaP, AlGaInP, AlGaPfor green and ZnSe, InGaN, SiC for blue). In either case, the directemission/direct view of the micro-LED display promises a six-foldimprovement or more in power efficiency.

Although the basic technology to realize micro-LED based displays iswell known, numerous manufacturing and quality control challenges exist.One of these is functionally testing millions of micro-LED deviceswithin the manufacturing process in a cost-effective and efficientmanner before the pixels are committed to use. It is therefore desiredto enable functional testing without direct electrical contact and in amanner compatible with micro-LED large-scale manufacturing processes.Further details of the present invention can be found throughout thepresent specification and more particularly below.

Embodiments of the present invention describe LED device fabricationprocesses and manners of functionally testing LED devices without directelectrical contact. In particular, some embodiments of the presentinvention may relate to manners of functionally testing high-brightnessLED, medium power LED, low-power LED and micro LED devices.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions, andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment”means that a feature, structure, configuration, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the invention. Thus, the appearances of the phrase “in oneembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “spanning”, “over”, “to”, “between” and “on” as used hereinmay refer to a relative position of one layer with respect to otherlayers. One layer “spanning,” “over” or “on” another layer or bonded“to” or in “contact” with another layer may be directly in contact withthe other layer or may have one or more intervening layers. One layer“between” layers may be directly in contact with the layers or may haveone or more intervening layers.

Certain embodiments of the invention describe an LED device assembly inwhich an LED device structure layer is transferred from a supportsubstrate and bonded to a pickup plate assembly before furtherprocessing. In accordance with embodiments of the present invention, theC²I functional testing step can be applied either before the transfer orafter one or more transfers. For the purposes of simplifying the variouspossible configurations wherein the plurality of the LED structures istransferred and possibly bonded onto a different substrate, the targetsubstrate shall be called a support substrate in each case. For example,the substrate that supported the LED structures during MOCVD growth isalso called a support substrate, however after release and attachment toa pickup plate, such a plate and any other substrate or plate used tomechanically support the LED device layer will also be called a supportsubstrate. If a pickup plate is used, common electrical contact can beaccomplished using an electrically conducting material film between thetransferred LED device structures and the rest of the pickup plate. Asfurther described below, the common contact can also be accomplishedusing a second dielectric layer and optional voltage waveform source. Insome cases, the pickup plate material would also have a degree ofcontrollable tackiness to allow LED device pickup and transfer inproduction. Additionally, the support substrate can be a flexible sheetsuch as a plastic film and the C²I can be used to test device structureson individual sheets or as part of a roll-to-roll manufacturing process(for example in an Organic-LED or OLED manufacturing process). The termsupport substrate will be generally used to connotate its role asmechanical support and will be the substrate described as part of (C²I)functional testing apparatus throughout this description.

Although the embodiments of this invention describe singulated andnon-singulated LED device structures, additional electronics such asactive-matrix addressing circuitry and support electronics can bepresent. Applying design for testability approaches and using thegeneral ability of the C²I functional test methods could successfullytest complex interconnected LED structures. For purposes of thisinvention, specific descriptions of the LED structures shall be taken asmerely examples and the test methods is to be understood as generallyapplicable to test singulated, non-singulated, lateral and vertical LEDstructures with or without integrated addressing and other supportelectronics.

Depending on the specific embodiment of this invention and the point inthe manufacturing process C²I functional testing is made, the supportsubstrate can be transparent and have additional coatings. These eitherdirectly support the test process or exist as part of the requirementsof the specific LED manufacturing process step as will be described inmore detail below.

Referring to FIG. 1, a representative LED 104 comprises of depositedlayers that form a n-type cathode layer 100, an active layer (usually aMulti-Quantum Well or MQW series of sub-layers) 101 and a p-type layer102 and p-contact 103. This LED structure is simplified and manyadditional layers such a buffer layers, blocking layers, n-contactlayer(s) and the like are not shown for simplicity. Electrically, theLED would be contacted through layer 103 (or contact 106) as the anodeand through layer 100 (or contact 105) as the cathode. In some LEDstructures, the n and p layers can also be contact layers and thus canbe named interchangeably for purposes of this invention unlessspecifically described otherwise. Passing current through the LED deviceusing a forward (positive voltage) bias from anode to cathode wouldgenerate light from radiative recombination processes from carriersflowing through the active region. The design of the active layer 101 isoptimized for maximizing radiative recombination processes that emitlight. Reverse biasing the LED structure will not generate light.Limiting reverse bias voltage is important with LEDs to avoid damagingor destroying the device through a process called breakdown. Within asafe reverse bias region, small leakage currents flow through thedevice.

In LED manufacturing, the LED devices are made in mass-production usingmethods similar to substrate-based mass-production processes common inthe semiconductor industry. Referring to FIG. 2, the LED structuredescribed in FIG. 1 is deposited onto a suitable growth or supportsubstrate 201 to make an LED substrate 200. Depending on the type,quality and color of the LED desired, different substrate material typescan be used. Examples are GaP, GaAs, GaN substrates or heteroepitaxialgrowth substrates such as sapphire and silicon carbide (SiC) are alsopossible. Layer-transferred semiconductor layered template substratesare yet another type of growth substrate. The LED structure is thengrowth to develop a lower contact 202 (n-type or cathode in thisexample), active region 203 and upper contact 204 (p-type or anode inthis example).

The LED substrate of FIG. 2 contains multiple, non-singulated LEDstructures. Isolation of individual LED devices of the desired size andfunction can be made within the LED manufacturing sequence using processsteps such as etching, lithography, passivation and deposition.Referring to FIGS. 3A and 3B, the desired LED devices can be isolatedwhile residing on support substrate 301 using processes such as etchingto form for example a trench 308. If these etch structures (sometimescalled “streets”) are made over the substrate to form individuallyisolated structures such as square devices, a high number of LED devices309 are electrically isolated and available for release and packaging.In this example, the trench 308 does not etch through the bottom commoncontact layer 302 and can thus be connected to a common potential 310.Each LED device 309 can thus be individually contacted using a voltagesource 306 to the p-layer 304 and p-contact layer 305. Light 307 canthen be measured from the contacted device to evaluate itsfunctionality. In this example, a top emitting LED structure is shownwhere the top contact 305 could be a transparent electrode such asIndium Tin Oxide (ITO). Other structures are possible such as a bottomemitting structure. In this case, the support structure would bepreferably transparent and the p-contact layer would be a lightreflecting layer such as a metal layer. The LED would thus be tested bymeasuring the light escaping from the support substrate. Although theabove was described as preferred embodiments to maximize the lightcapture, it would be possible to measure indirectly scattered orreflected light from LEDs even if the light measurement was done, forexample, above the LED in a bottom emitting LED structure. Of course,there can be other variations, modifications, and alternatives.

FIG. 4 shows a support substrate 401 where the LED devices are still notisolated. If the top contact layer 405 has a limited conductivity (suchas an ITO layer with a relatively high film sheet resistivity),functional testing could still be accomplished despite a short 408present nearby. Contacting a point on the surface using a voltage source406 would develop a current through the top contact 405, p-layer 404,active layer 403, n-layer 403 to common contact 402. The relatively highresistance to the neighboring short 408 could allow light emission 407to occur. Using a field plate in lieu of this direct contact exampleaccording to an embodiment of this invention would allow non-singulatedLED layer testing. Dark (non-emissive) or weakly emissive areas would bean indicator of LED layer functional yield at an early stage of the LEDmanufacturing process. The efficacy and spatial resolution of thisalternative embodiment would be a function of top layer sheetresistivity.

There is therefore a need to inject a current to excite individual LEDdevices or LED areas such as described in FIGS. 3 & 4 in a manner thatcan support large-scale manufacturing.

The invention has as its current injection device a field platecomprising of 3 elements: a mechanical support plate, an electrode and adielectric layer. Referring to FIG. 5A, the field plate 501 comprises ofa field plate support (top), electrode layer 502 connected to a voltagesource 503 and adjacent to one face of dielectric layer 504. Themechanical support plate can also be electrically conductive and onlyrequire a dielectric layer. Of course, there can be other variations,modifications, and alternatives.

The field plate electrode would be connected to voltage source 503 andthe open face of the dielectric layer 504 would form a capacitance perunit area of:

C′ _(FP)=ε_(o×)ε_(r) /t _(d)   (1)

-   Where-   C′_(FP) is the capacitance per unit area of the field plate (F/cm²)-   ε_(o) is vacuum permittivity (8.854×10⁻¹⁴ F/cm)-   ε_(r) is the relative permittivity of the dielectric layer    (dimensionless)-   t_(d) is the dielectric layer thickness (cm)    In an example, important material characteristics of the dielectric    layer material includes dielectric constant, dielectric breakdown    strength, resistivity, and optical transmissivity. For capacitive    coupled configurations, readily deposited dielectrics such as    silicon dioxide, silicon nitride and alumina (Al₂O₃) are of    particular interest. If a DC test configuration is desired, a    dielectric having limited current leakage would allow DC biasing if    coupled to the device using an appropriate gap medium also having    limited resistivity. In such a configuration, the field plate    dielectric can be optional where the field plate voltage can now be    directly coupled to the LED devices through the gap medium. Of    course, there can be other variations, modifications, and    alternatives.

Again referring to FIG. 5A, the field plate 501 would be placedsufficiently near LED support structure 505 with an n-contact bottomelectrode 506 connected to a common contact 507 and a plurality ofp-contact top electrodes 508. Although the voltage across each LEDdevice is shown in this description as being developed using a voltagesource 503 and common contact 507, the voltage source can bealternatively connected to the bottom or two voltage sources can beconnected, one to each of contacts 503 and 507. The effective LED devicedrive voltage would be the voltage difference between contacts 503 and507 for all voltage source configurations. For this invention, the term“close proximity” shall mean the open face of the field plate dielectriclayer 504 is placed in sufficient proximity to the open face of the LEDstructure contact surface 508 to allow a desired electrical couplingbetween the voltage source 503 to the top LED electrode surface 508. InFIG. 5A, this gap is shown as 509 and can be minimal with a limited gapor no gap. Gap 509 should be small enough to allow sufficient capacitivecoupling (for optimizing the current injection efficiency) and tominimize spatially defocusing the current injection effect. For topplate 501 with a relatively thick dielectric layer 504 and highervoltage levels on voltage source 503, a limited gap 509 is possible toallow non-contact testing without significantly lowering couplingefficiency (C_(EFF)′˜C_(FP)′). For the rest of this description, gap 509will be assumed to be zero and thus C′_(EFF) will be made equal toC′_(FP).

The electrical analogue of the structure made by assembly 500 is shownin FIG. 5B. Voltage source 510 (503 in FIG. 5A) is connected to aneffective capacitor C_(EFF) 511 connected to LED device 512 having a topsurface area A_(EFF). A voltage change will impress a current I_(LED)onto LED device 512. For this example, isolation of LED devices with acommon bottom contact is assumed. The effective capacitance C_(EFF) issimply the series capacitance of the field plate dielectric layer withthe capacitance of gap 509, both of area A_(EFF):

C′ _(gap)=ε_(o×)ε_(r) /t _(gap)   (2)

-   Where-   C′_(gap) is the capacitance per unit area of the gap (F/cm²)-   ε_(o) is vacuum permittivity (8.854×10⁻¹⁴ F/cm)-   ε_(r) is the relative permittivity of the gap medium (dimensionless)-   t_(gap) is the gap thickness (cm)    and

C _(EFF) =A _(EFF)×(C′ _(FP) ×C′ _(gap))/(C′ _(FP) +C′ _(gap))   (3)

C′ _(EFF)=(C′ _(FP) ×C′ _(gap))/(C′ _(FP) +C′ _(gap))   (4)

-   Where-   C_(EFF) is the effective LED device coupling capacitance (F)-   C′_(EFF) is the effective LED device coupling capacitance per unit    area (F/cm²)-   A_(EFF) is the effective LED device area (cm²)    For the rest of this description, gap 509 will be assumed to be zero    and thus C′_(EFF) will be made equal to C′_(FP).

The current I_(LED) 513 and current density J_(LED) are readilycalculated as:

I _(LED) =C _(EFF) ×dV/dt   (5)

J _(LED) =C′ _(EFF) ×dV/dt   (6)

Where dV/dt is the voltage rate of change between the voltage source 510and the common electrode 507 in FIG. 5A (or the cathode contact in FIG.5B). For this particular embodiment, LED 512 is connected anode tocathode however cathode to anode injection is possible by reversing allvoltage polarities.

FIGS. 6A and 6B show the voltage and current waveforms that would form ameasurement sequence according to a preferred embodiment of thisinvention. There are at least 2 phases in the measurement sequence, thecurrent injection phase I (at time t₀ to t₁) and the discharge phase III(at time t₂ to t₃). A voltage holding phase II has been added to allowfor enough time for the camera integration window to close before phaseIII commences however this can be very short and may not be necessary.The phases are described in more detail below and assumes null voltageat all point before t₀.

Referring to FIG. 6A, a voltage source-time plot 600 shows the voltagesource waveform. At time t₀, phase I begins with a positive ramp dV/dt|₁from 0 to V₁ from time t₀ to t₁. This ramp will inject a current I_(LED)into a LED of area A_(EFF) according to equation (5) and a correspondingcurrent density J_(LED) according to equation 6. At time t₁, the voltageis held at this voltage V₁ until time t₂. From time t₂ to t₃, thevoltage is lowered back to a zero-voltage state using a negative rampdV/dt|₂. At time t₃, another measurement sequence can then commence.

FIG. 6B shows the corresponding current I_(LED) waveform during themeasurement sequence from the driving waveform of FIG. 6A. During phaseI, a near constant I_(LED) current will flow through the LED deviceaccording to equation (5). Light 602 will be emitted during Phase I.During Phase II, I_(LED) and light emission will drop to 0. To measurethe light output of a particular LED device among a plurality of devicessimultaneously being excited by the field plate, an integrating camerathat can capture the light from one or more LED devices is used. Imageprocessing can generate a value proportional to specific LED deviceslocated within the camera's field of view. This value would be in turnproportional to the optical energy integrated over phase I. It istherefore desirable to start an image sensor integrating time periodfrom a point slightly before t₀ to an end point slightly after t₁. Thiswill ensure the camera integrating sensor will capture the completelight pulse emanating from the LED structures during phase I.

FIGS. 7A and 7B show two embodiments of the invention. The figures showtop and bottom camera placement that can intercept at least a portion ofthe plurality of LED devices being excited by the measurement sequencethrough the field plate. Referring to FIG. 7A, a test configuration 700is shown with a transparent field plate assembly 702 including a fieldplate electrode 703 and dielectric layer 704. Electrode 703 is connectedto a voltage source 705. This assembly is placed in close proximity tothe LED device support substrate 701 supporting a plurality of LEDdevice 707 connected to a common contact 706. A camera 708 is placedabove the field plate assembly 702 to perform the functional test. FIG.7B shows an alternative test configuration 709 where the camera isplaced below the support substrate. In this configuration, the supportsubstrate and intermediate layers to the LED device structures must betransparent to allow the light to reach the camera.

Optical power generated by an LED is related to electrical power flowingthrough the LED by the external quantum efficiency η_(EXT) orP_(opt)=η_(EXT)×P_(elec). The parameter η_(EXT) is in turn quitesensitive to the current density and other device characteristics suchas light extraction efficiency. The optical power of an LED device isthus related to electrical power as:

P _(opt)=η_(EXT) ×P _(elec)=η_(EXT) ×I _(LED) ×V _(F)=η_(EXT) ×C _(EFF)×dV/dt×V _(F)   (7)

-   Where-   P_(opt)=LED optical power (W)-   η_(EXT)=LED external quantum efficiency-   V_(F)=LED forward voltage drop (V)    Over a time period Δt=t₁−t₀ (phase I):

E _(opt)=η_(EXT) ×C _(EFF) ×dV/dt×V _(F) ×Δt   (8)

-   Where-   E_(opt)=LED optical energy emitted during phase 1 (J)

According to equation 8, the integrating camera will measure a valueproportional to each measured LED's external quantum efficiency.Changing the voltage ramp value will have the effect of selecting adifferent current density according to equation 6. By plotting Eopt as afunction of ramp value to V₁, a plot of light energy (related toη_(EXT)) as a function of J_(LED) can be generated. This capability canbe particularly useful to measure low current density performance ofmicro-LED devices. Micro-LED devices are typically driven at very lowvalues of 0.001-1 A/cm² and are more sensitive to drop in externalquantum efficiency at these low levels due to non-radiativerecombination processes.

During phase III, a negative dV/dt ramp allows the voltages to bereturned to zero to reset the system for another measurement. Duringthis phase, the LEDs will be reverse biased and will discharge C_(EFF)using reverse bias leakage current. So as not to reverse bias the LEDsto a voltage level that can cause damage, the negative voltage ramp mustbe sufficiently slow to keep all devices within a safe reverse biasvoltage range. Such a range can be selected depending on the type anddesign of the LEDs to be tested. As merely an example, reverse biasleakage current density for GaInN LEDs can be estimated using a paperentitled “Transport mechanism analysis of the reverse leakage current inGaInN light-emitting diodes”, Q. Shan & al., Applied Physics Letter 99,253506 (2011). FIG. 2 shows a −5V reverse bias leakage current ofapproximately 1.5×10⁻⁷ A on a 1 mm² LED device at room-temperature. Thiscorresponds to 15 μA/cm². This reverse bias leakage current density willbe used to calculate values and parameters for specific C²I examplesdescribed below.

Suitable integrating cameras must satisfy the following criteria:

-   -   a. Pixel sensitivity and dynamic range (allows LEDs to be        accurately measured through the operating range of interest        without excessive dark noise and signal saturation).    -   b. High pixel density and frame rate (increases throughput and        parallel LED measurement).    -   c. Global shutter and flexible triggering (all pixels must be        triggered and integrate in over the same time period).        One example of an industrial camera meeting these criteria is        model GS3-U3-23S6M-C from PointGrey Research Inc., Richmond, BC,        Canada. The camera is a 2.3 megapixel (1920×1200) monochromatic        camera with global shutter, 5 μsec to 31.9 sec exposure range,        over 160 frames per second rate, 1/1.2″ sensor format, 12-bit        digitization, 5.86 μm pixel size, 72 dB dynamic range, 76%        quantum efficiency (525 nm), an e− saturation capacity of about        32,000 electrons and temporal dark noise of about 7 e−. Used        singly or in a matrix arrangement where n×m cameras would be        used to measure a larger field plate area simultaneously, the        camera will have the capability to measure numerous LED devices        with the requisite accuracy.

For the following examples, a field plate with a 3 μm silicon dioxidedielectric layer is assumed (ε_(r)=3.9). This dielectric material iscommonly used and can be sputtered, grown or deposited on numerousmaterials. The thickness was selected to be sufficiently thin to allowtesting of micro-LED devices down to 10 μm×10 μm or less and can supporta voltage exceeding about 1500 volts before breakdown. C′_(FP) would be1.15 nF/cm².

A value for V₁ of 500V is assumed (see FIG. 6A). With these assumptionsand parameter selections, the light pulse energy per LED can besimplified as:

E _(opt)=η_(EXT) ×C _(EFF) ×ΔV×V _(F)   (9)

For the parameters selected, FIG. 8 shows the current density selectedfor a voltage ramp time period. For example, the LED would be driven at0.01 A/cm² if the field plate voltage was driven from zero to +500 Voltsin approximately 60 μsec (phase I). The camera shutter would be openedslightly before the ramp begins (for example, 10-50 μsec before t₀) andwould be opened slightly after the end of phase I (for example, 10-50μsec after t₁). Apart from ensuring that the phase I LED light pulse isfully integrated within the camera shutter time window, excessiveintegration time should be avoided as this will tend to raise the noisefloor of the camera. Phase II can be selected to end just as theintegrating shutter closes.

To safely recover during phase III, equation 6 is utilized with thecurrent density selected to be approximately equal to the leakagecurrent density. For example, utilizing a target leakage current densityof 10 μA/cm² (a little lower than the expected leakage of 15 μA/cm²) andΔV=500V, equation 6 predicts a minimum Δt of almost 60 msec. Thiscorresponds to a measurement repetition rate of about 16 frames perseconds for injection current densities of 0.0005 A/cm² or above.

Faster recovery rates are possible using an external light sourceilluminating the LED device area at a sufficiently short wavelength toexcite carriers within the LED device junction. The enhancedphotocurrent leakage would allow faster phase III recovery withoutdeveloping high reverse bias conditions.

To estimate the signal achievable with this measurement approach and thearea covered by one camera, the following additional parameters areassumed:

-   -   a. GaN LED (about 410 nm emission & 65% camera quantum        efficiency)    -   b. V_(F) about 3V    -   c. E_(opt)=170 nJ/cm2 (η_(EXT)˜10%)

At about 3 eV per photon, approximately 3.5×10¹¹ photons/cm² are emittedduring phase I. The number of corresponding photo-electrons that can begenerated within the camera would be 0.65×3.5×10¹¹ photons/cm² or2.3×10¹¹ photo-electrons/cm² (assuming sensor to field plate 1:1magnification and 100% collection efficiency). At this magnification, a5.86 μm pixel size would still capture over 78,000 electrons, more thantwice the pixel saturation capacity. A lower V₁ voltage could beselected if a lower integrated photo-electron count per camera pixel isdesired.

Using a 0.5× collection lens (model #62-911 TECHSPEC™ large formattelecentric lens manufactured by Edmund Optics, Barrington, N.J., USA)as an example, a roughly 21 mm×16 mm LED substrate area will be imagedonto the image sensor, although there can be variations. The workingdistance is 175 mm and the entrance aperture is roughly 90 mm, resultingin about 1.7% total collection efficiency compared to 4π sr. Theexpected photo-electrons collected would be 2.3×10¹¹photo-electrons/cm²×(5.86 μm×2)²×0.017=5.3 ke⁻/pixel. This is wellwithin the sensor's dynamic range and signal averaging or increasing V₁could further improve signal quality if desired. Of course, there can beother variations, modifications, and alternatives.

The imaging of the field plate to the camera sensor area is thus less afunction of the available signal than the number of pixels allocated perLED device. For a larger LED device measuring 250 μm on a side, lessmagnification is necessary. Assuming a 2×2 pixel area to cover each LEDdevice for accurate metrology, one camera could measure 960×600 LEDdevices, 240 mm×150 mm field plate area or more than the area of a 6″support substrate. In this example, V₁ could be reduced to less than100V and perhaps lower while still maintaining excellent signal to noiseratio. If the light pulse energy is too high, a neutral density filteror other absorption filter can be placed between the emitting surfaceand the camera to avoid camera saturation.

For a micro-LED application with 10 μm×10 μm LED device size, the same960×600 LED devices per sensor would be measured or about 9.6 mm×6 mmfield plate area. A step and repeat system with approximately 16×25steps would allow testing of a 6″ micro-LED substrate containing over170 million devices. If a single measurement per LED device issufficient, a synchronized image capture with a moving camera or camerascould decrease test time to less than 1 minute or even a few seconds.For example, a 16 frame per second capture rate would allow a full 6″substrate to be functionally tested in about 25 seconds. That wouldcorrespond to over 9 million LED devices tested per second, far fasterthan probe cards and individual test methods.

In a preferred embodiment, FIG. 9 shows a substrate-sized field platecan be attached to a support substrate using vacuum to make an assembly900 suitable for functional test. The field plate 901 is placed on theLED device support substrate 902 with a compliant vacuum seal 903 placedon the outside peripheral area to maintain a level of vacuum between thefield plate and the LED device support substrate. The air is thenevacuated in the space between the plates using vacuum port 904. Theplates would be pressed together at up to atmospheric pressure tominimize the gap in a uniform manner, thus optimizing the effectivefield plate coupling capacitance C_(EFF). A support substrate exchangemechanism could exchange substrates to be tested under the field plateby cycling port 904 between vacuum and vent conditions. A camera 905that measures above the field plate is shown in this embodiment. Insteadof vacuum, a fill and evacuation port can supply a suitable liquidcoupling medium within the gap. Of course, there can be othervariations, modifications, and alternatives.

In another embodiment, FIG. 10 shows an assembly comprising of a smallerfield plate 1000 and camera 1001 placed above an LED device supportsubstrate 1002. The field plate/camera assembly is moved in successivemove/measure steps 1003 to measure the complete substrate 1002. Ofcourse, there can be other variations, modifications, and alternatives.

Electrical simulations of the measurements sequence showing the mainphase 1 and 3 waveforms are shown in FIGS. 11-14. The system beingsimulated is as follows:

-   -   1. Field plate: 3 μm silicon dioxide, C′_(EFF)=1.15 nF/cm2    -   2. 10 μm×10 μm LED device size: 1.15 fF C_(EFF), 15 pA reverse        leakage current    -   3. 0.01 A/cm² current density test point    -   4. V₁=500V (60 μsec ramp time to achieve 0.01 A/cm² current        density injection)    -   5. 60 msec measurement repetition rate    -   6. LED device is a standard diode capable of reverse leakage        current of about 10 pA

The program used is a SPICE circuit simulator called Micro-Cap version11 from Spectrum Software (Sunnyvale, Calif.). One 10 μm×10 μm LEDdevice was simulated under the above conditions. FIG. 11 shows thecircuit diagram where C_(EFF)=1.15 fF driven by a voltage generator V2.This generator was programmed to ramp from 0 to +500V in 60 μsecfollowed by a 60 msec ramp down from +500V to 0V. Voltage source V3 isunconnected but was programmed to show an example of a desired camerashutter window. In this example, the shutter is opened 10 μsec beforephase I and closes 10 μsec after phase I.

FIGS. 12A-D show the Phase 1 waveforms with the voltage source V2 (FIG.12A), LED device forward bias (FIG. 12B), LED device forward current(FIG. 12C) and the camera shutter control signal from voltage source V3(FIG. 12D). Referring to FIG. 12D, the camera integrator shutter opens10 μsec before the start of the voltage (at time +10 μsec on the timeaxis). At time +20 μsec on the time axis, the voltage source starts toramp towards +500V (time t₀). During this phase I until time +80 μsec,the LED device is biased at +10 nA (FIG. 12C) at a forward bias ofapproximately +250 mV (FIG. 12B). This corresponds to 0.01 A/cm² currentdensity as desired. After time +80 μsec, the voltage ramp stops and theLED current drops to zero. At time +90 μsec, the camera shutter closes,completing its integration of the light pulse that was generated duringphase I. The voltage source will now start a slow discharge at a targetleakage current of −10 pA. FIGS. 13A-D show the same voltage and currentpoints during the phase III discharge lasting about 60 msec. FIG. 13Cshows a −10 pA discharge current that allows C_(EFF) to discharge over60 msec from +500V to 0V safely. After the voltage source is returned tozero at about +60 msec, a new measurement sequence is initiated. FIGS.14A-D show a longer time axis (200 msec) showing 4 measurementsequences.

A direct common contact also allows DC biasing and functional testingconfigurations. FIG. 15 shows an embodiment where the LED devices can bebiased in a DC mode only or in combination with a time varying AC mode.Referring to FIG. 15A, the field plate 1501 comprises of a field platesupport (top), electrode layer 1502 connected to a voltage source 1503and adjacent to one face of an optional “leaky” dielectric layer 1504.The mechanical support plate can also be electrically conductive andonly require the optional “leaky” dielectric layer 1504. Of course,there can be other variations, modifications, and alternatives.

The field plate electrode is connected to voltage source 1503 and theopen face of the optional “leaky” dielectric layer 1504 form acapacitance per unit area of:

C′ _(FP)=ε_(o×)ε_(r) /t _(d)   (10)

-   Where-   C′_(FP) is the capacitance per unit area of the field plate (F/cm²)-   ε_(o) is vacuum permittivity (8.854×10⁻¹⁴ F/cm)-   ε_(r) is the relative permittivity of the dielectric layer    (dimensionless)-   t_(d) is the dielectric layer thickness (cm)    The dielectric would have a resistivity of ρ_(d), selected to allow    the desired biasing of the LED devices in a DC bias configuration.    The time constant driving the bias response time is    ε_(o)×ε_(r)×ρ_(d). The effective resistance can be calculated as    follows:

R′ _(FP)=ρ_(d) ×t _(d) (ohms-cm²)   (11)

-   Where-   R′_(FP) is the resistance for a unit area of the field plate    (ohms-cm²)-   ρ_(d) is the resistivity of the field plate dielectric layer    (ohm-cm)-   t_(d) is the dielectric layer thickness (cm)    In an example, the leaky dielectric layer can be generally described    as a layer with a reasonably high relative dielectric constant,    resistivity on the order of 1 Mohm-cm or higher and a sufficiently    high dielectric breakdown field strength. Type II DI (deionized)    water meets these criteria with a dielectric constant of 81,    resistivity of 1 Mohm-cm and a breakdown field strength exceeding 13    MV/cm. In other examples, the layer can be a slightly conductive    doped glass/ceramic, plastic or the like. If a small relative    dielectric constant of about 1 is acceptable, an air layer with a    voltage within a gap could become slightly conductive by weak    ionization to accomplish the function of a “leaky” dielectric layer.

Again referring to FIG. 15A, the field plate 1501 would be placedsufficiently near LED support structure 1505 with an n-contact bottomelectrode 1506 connected to a common voltage source 1507 and a pluralityof p-contact top electrodes 1508. Although the voltage across each LEDdevice is shown in this description as being developed using a voltagesource 1503 and common voltage source 1507, a voltage source can bealternatively connected to either contact 1502 or 1506. The effectiveLED device drive voltage would be the voltage difference betweencontacts 1503 and 1507 for all voltage source configurations. For thisconfiguration, the open face of the field plate dielectric layer 1504 orthe electrode contact 1502 is placed in sufficient proximity to the openface of the LED structure contact surface 1508 to allow a desiredelectrical coupling between the voltage source 1503 to the top LEDelectrode surface 1508. In FIG. 15A, this gap is shown as 1509 and canbe minimal with a limited gap. Gap 1509 should be small enough to allowsufficient capacitive and resistive coupling (for optimizing the currentinjection efficiency) and to minimize spatially defocusing the currentinjection effect for the selected gap medium.

The electrical analogue of the structure made by assembly 1500 is shownin FIG. 15B. Voltage source 1510 (1503 in FIG. 15A) is connected to eachLED device 1512 having a top surface area A_(EFF) through two effectivecapacitors, one representing the optional “leaky” field plate dielectric1504 (C_(FP) 1511) and one representing the gap dielectric medium 1509(C_(GAP) 1513). Each capacitor is shunted by a resistor representing theleakage path through the field plate dielectric R_(FP) 1514 and gapmedium R_(GAP) 1515. A voltage source 1516 (1507 in FIG. 15A) connectedto the bottom common contact 1506 completes the electrical circuit. Avoltage change and level will impress a current I_(LED) 1517 onto LEDdevice 1512. The effective capacitance C_(EFF) is simply the seriescapacitance of the field plate dielectric layer with the capacitance ofgap 1509, both of area A_(EFF):

C′ _(gap)=ε₀×ε_(r) /t _(gap)   (12)

-   Where-   C′_(gap) is the capacitance per unit area of the gap (F/cm²)-   ε_(o) is vacuum permittivity (8.854×10⁻¹⁴ F/cm)-   ε_(r) is the relative permittivity of the gap medium (dimensionless)-   t_(gap) is the gap thickness (cm)    and

C _(EFF) =A _(EFF)×(C′ _(FP) ×C′ _(gap))/(C′ _(FP) +C′ _(gap))   (13)

C′ _(EFF)=(C′ _(FP) ×C′ _(gap))/(C′ _(FP) +C′ _(gap))   (14)

-   Where-   C_(EFF) is the effective LED device coupling capacitance (F)-   C′_(EFF) is the effective LED device coupling capacitance per unit    area (F/cm²)-   A_(EFF) is the effective LED device area (cm²)    The gap medium shunt resistor is calculated as:

R′ _(GAP)=ρ_(gap) ×t _(gap) (ohms-cm²)   (15)

-   Where-   R′_(GAP) is the resistance for a unit area of the gap medium    (ohms-cm²)-   ρ_(gap) is the resistivity of the gap layer (ohm-cm)-   t_(gap) is the gap layer thickness (cm)    and the effective shunt resistor is calculated as:

R _(EFF)=(R′ _(FP) +R′ _(GAP))/A _(EFF)   (16)

-   Where-   R_(EFF) is the effective coupling shunt resistance (Ohms)

An example of the DC injection functional test mode is explained hereusing the structure of FIG. 16. The test configuration is as follows:

-   -   1. Device size: 25 μm×25 μm    -   2. Field plate has no dielectric layer    -   3. Gap is 25 μm with type II DI water (>1 Mohm-cm)    -   4. Target DC bias of 10 mA/cm² set by bottom electrode voltage        source and bias load resistor R_(L)    -   5. AC (pulsed) operation driven by the top field plate voltage        source through an external coupling capacitor C_(C)    -   6. Test area 5 cm²        The field plate 1601 is placed sufficiently near LED support        structure 1602 with an n-contact bottom electrode 1603 connected        to a common ground contact 1604 and a plurality of p-contact top        electrodes 1605. For each LED device, the gap medium coupling        capacitance is 18 fF and its shunt resistance is 400 Mohm. The        relatively large coupling capacitance is made possible using DI        water in gap 1606 which has a high relative dielectric constant        of 81. The AC and DC components of the injected current within        each LED device is denoted by current 1617.

The DC bias is set by adjusting voltage source 1607 to a preset positivepotential V_(pos) to bias the LED devices through the load resistorR_(L) 1608, field plate contact 1609, gap medium 1606, through the LEDdevices to bottom contact 1604. To achieve the DC bias point of 10mA/cm², a total current of 50 mA must flow through R_(L). If R_(L) ischosen as 100 kohm to allow an efficient coupling of an AC pulse throughcoupling capacitor C_(C) 1610 roughly 5 kV positive bias V_(pos) isneeded. The voltage drop through the DI water is about 25V while the LEDrequires 2.5-3.5V to turn on. Note that by varying the bias level, agraph of the output light level versus DC current density bias can bemeasured. Pulsing the DC bias synchronously with the camera capture timewould allow signal averaging and multiple device bias set points to bemeasured.

A pulsed signal response can be combined by pulsing source 1611 throughcoupling capacitor 1610. Assuming a C_(L) of 1 nF, a fast pulse cancouple charge into the LED devices before R_(L)×C_(C) relaxation canlower the charge coupling efficiency. For 100 kohm and 1 nF devicevalues, the system relaxation time constant is 100 μsec, a valuesufficiently long to ensure that the charge can be transferred to theLED devices and converted to a measurable pulse of light. In thisexample, the total DI water coupling capacitance is about 14 nF. If a100 mA/cm² bias is injected for 5 μsec, a total charge of about 3 pC perLED device or 2.4 μC for the test area must be delivered. Since the DIcoupling capacitor is about 14 nF, the effective coupling capacitancewould still be about 1 nF. The voltage pulse magnitude required ofsource 1611 would be 2400 volts ramped in 5 μsec. This way, both DCbiasing and AC pulsed functional test can be made of an LED devicesubstrate with a bottom common contact. Of course, there can be otherbias and pulsing configurations, variations, modifications, andalternatives.

Although this invention has been described with a common contactexisting under the LED devices, other device of injecting current arepossible. FIG. 17A shows another embodiment 1700 where an analogue tothe field plate 1701 is present within support substrate 1702 below theplurality of LED device structures such as LED device 1703. Under thelowest LED device structure layer (the n-layer in examples described inthis invention), a dielectric layer 1704 and electrode 1705 completesthe support substrate capacitive coupling device. Electrode 1705 isconnected to a voltage source 1706. The field plate is connected to aseparate voltage source 1707 and field plate electrode 1708. In thisexample, a camera 1709 is placed above the field plate to capture thelight emission response of the plurality of LED devices under test. Inthis example, the isolation between devices is shown to be complete,however this method would still function with or without full isolationof the n-layer. FIG. 17B shows the equivalent circuit 1711 of thiscapacitively coupled support substrate configuration. The only change isthe insertion of a second coupling capacitor C_(EFF2) below each LEDdevice cathode. The resulting circuit can be made to operateequivalently and be effective in performing C²I functional test. Forexample, assuming a support substrate dielectric layer 1704 identical todielectric layer 1710 within the field plate, voltage source 1706 drivenidentically but negatively to voltage source 1707 (0 to −500V for source1706 and 0 to +500V for source 1707), the measurement system 1700 wouldperform essentially identically to a common contact support substrateconfiguration.

In yet another embodiment, C²I functional testing could also be appliedto a modification of the test configuration of FIG. 17A that eliminatesthe need for a buried electrode within the support substrate. In thisembodiment, the dielectric property of the support substrate itself isused to inject current through the LED devices. For example, a quartz,sapphire, glass or plastic support substrate could serve as dielectric1704 in FIG. 17A. FIG. 18 shows a specific embodiment 1800 of thisconfiguration. A support substrate 1801 having adequate dielectricproperties and thickness containing a plurality of LED devices on itssurface such as LED device 1802 is placed on top of an electrode 1803connected to a voltage source 1806. Although not shown specifically, agap can exist between electrode 1803 and support substrate 1801,allowing non-contact operation of the back side of the support substrateif desired. Although not shown specifically, an additional dielectriccan also be interposed between electrode 1803 and support substrate1801. The gap (and optional dielectric covering electrode 1803) willmodify C_(EFF2) in a similar manner as the top field plate to device gap509 modified C_(EFF) & C_(EFF)′ using equations 2-4. Field plate 1804having a dielectric layer 1805 and electrode 1806 connected to a secondvoltage source 1807 completes the C²I functional test circuit. A camera1808 placed above field plate 1804 is shown in this embodiment. Theequivalent electrical circuit would be similar to FIG. 17B except thatthe value C_(EFF2) is likely substantially smaller due to the thicknessof the support substrate. For example, a support substrate made ofsapphire (ε_(r)˜10) of 500 μm in thickness, C′_(EFF2) will beapproximately 18 pF/cm², about 65 times smaller than C_(EFF1). A fastervoltage ramp and/or a larger voltage value for V₁ could compensate forthis loss of coupling efficiency. For example, field plate voltagesource 1807 could be driven from 0 to +500V while the support substratevoltage source 1804 could be driven at 0 to −32.5 kV (−500V×65=−32.5kV). The electric field strength within the sapphire support substratewould be 0.65 MV/cm, well below its breakdown strength of approximately1 MV/cm. Driven in this fashion, the LED devices would be drivensubstantially equivalently and allow C²I functional test without aburied contact within the LED device support substrate. High-voltagewaveform generators to drive electrode 1803 can be realized using IGBT,MOSFET, or thyristor devices. High-voltage switches capable of switchingup to 36 kV are model number HTS-361-01-C (36 kV, 12 A max current) andmodel number HTS-361-200-FI (36 kV, 2000 A max current) from BelkeElectronic GMBH (Kronberg, Germany). Programmable waveform shapingcircuits could slow the fast voltage change to a voltage ramp meetingthe desired C²I functional test properties. For a 6″ substrate, thetotal capacitance would be about 3.2 nF and at 16 measurements perseconds, the ½ CV²f power would be about 27 Watts and the averagecurrent would be 830 μA, safely within normal operating specificationsfor commercially available high-voltage switches. For the HTS-361-200-FI2000 A capable switch, current density C²I measurements as high as 11A/cm² could be performed. Of course, there can be other variations,modifications, and alternatives.

FIG. 19 show capacitive current injection variants A, B and C which mayinclude up to 3 voltage sources to energize LED structure 1900. VariantA is a vertical LED structure where the top LED structure areacorresponds to the top electrode (anode) and the bottom LED structurearea corresponds to the bottom electrode (cathode). C_(top-a) istherefore equal to C_(EFF1) and C_(bot) is equal to C_(EFF2) of FIG.15B. Lateral LED structures such as the LED device 1901 with a small toparea etched to expose the n-layer 1902 (cathode) while the balance ofthe area contains the active layer, top p-layer and p-layer contact 1903(anode). In variant B, the top anode contact is accessible by aneffective capacitance C_(top-b) while cathode contact area 1902 isaccessible by an effective capacitance C_(bot-b). The cathode area isalso accessible through the support substrate 1904 via capacitanceC_(bot) and bottom electrode 1905. If the LED device has a total topsurface area equivalent to variant A, C_(top-b)+C_(bot-b) will equalC_(top-a) and each capacitance value will be proportional to its surfacearea. For example, if the top anode area is 75% of the total area (andthe top cathode area is 25% of the total area), C_(top-b)=0.75×C_(top-a)and C_(bot-b)=0.25×C_(top-a). There is no change in C_(bot) since thetotal bottom area is not modified, however, the active area is smaller(75% in this example) due to the MESA etch contacting method. Effectivecurrent density injection value calculations would require active areato total area ratio correction. Variant A has a smaller top cathodecontact using a via contact. Except for differing capacitances andactive area to total area ratio values, this variant is similar tovariant B.

Using the lateral LED MESA structure of variant B as an example LEDstructure, FIG. 20 shows a test assembly 2000 with three possiblecontacting options of three separate LED devices 2001 using a top fieldplate 2002 atop of the devices on the support substrate 2003 with abottom electrode 2004. Note that electrode 2004 can be an electrodeassembly including an overlying dielectric and may be placed inproximity to support substrate 2003. C_(bot) would be modified toinclude such additional dielectric layer(s) and gap.

In option A, the top field plate electrode 2005 covers the complete LEDdevice and is driven with a voltage ramp to voltage V₁. The bottomelectrode 2004 is driven by a negative voltage ramp to voltage V₂. Thetop field plate electrode 2005 is assumed to be driven using a positiveslope according to Phase I of FIG. 6A while the bottom electrode 2004 isassumed to be driven by a negative slope where V₂ is scaledappropriately by the capacitances as explained further below. Variant Bis a patterned field plate electrode where only an anode electrode 2006is present and injects current into the LED device through capacitanceC_(t-b). The top cathode capacitance is assumed to be negligible due tothe absence of an electrode above the top cathode contact area. VariantC has a patterned field plate electrode structure where the top anodecontact is capacitively coupled to field plate electrode 2007 using apositive ramp to voltage V₁ and the top cathode contact is capacitivelycoupled to field plate electrode 2008 using a negative ramp to voltageV₃.

Unless stated otherwise, zero net charge accumulation in the device(anode charge in=cathode charge out) and equal voltage ramp periods areassumed. Applying these bias conditions to the structures of FIG. 20 forphase I, the various relationships are as follows:

-   -   1. Variant A (lateral LED device, continuous field plate)

Injected current=C _(t-a) ×dV ₁ /dt

V ₂ =−V ₁×(C _(t-a) +C _(b-a))/C _(bot)

-   -   2. Variant B (lateral LED device, field plate electrode on anode        only)

Injected current=C _(t-b) ×dV ₁ /dt

V ₂ =−V ₁ ×C _(t-b) /C _(bot)

-   -   3. Variant C (lateral LED device, separate field plate electrode        on anode and cathode)

Injected current=C _(t-c) ×dV ₁ /dt

V ₁ ×C _(t-c) =−V ₂ ×C _(bot) −V ₃ ×C _(b-c)

For all three structures, J_(LED)=Injected current/active area, wherethe active area is the area of the MQW structure contacted by thedevice.

As merely examples to demonstrate the use of C²I measurement on lateraldevice structures, the LED structures of FIG. 20 are assumed with thefollowing common parameters and photo-collection configuration:

-   -   a. 25 μm×50 μm device size    -   b. 75% (anode & active area) and 25% (cathode) area split for        the lateral LED Device    -   c. Target J_(LED)=0.01 A/cm²    -   d. Sapphire support substrate thickness of 1 mm (ε_(r)=10)    -   e. Phase I voltage ramp time of 25 μsec    -   f. Field plate dielectric: 2 μm silicon nitride (ε_(r)=7.5)    -   g. C_(t-a)=C_(t-b)=C_(t-c)=31 fF    -   h. C_(b-a)=C_(b-c)=10.3 fF    -   i. C_(bot)=0.11 fF    -   j. Injected current=93.7 nA    -   k. E_(opt)=75 nJ/cm²    -   l. Photons/LED device emitted ˜1.5×10⁶ photons    -   m. Photo-electrons detected per LED device (65% quantum        efficiency, 90 mm lens aperture, 175 mm working distance)        ˜16,000 photo-electrons per C²I measurement cycle

VARIANT A EXAMPLE 1

-   -   a. V₁=+75V    -   b. V₂=−28.2 kV

VARIANT A EXAMPLE 2 (TOP FIELD PLATE GROUNDED CONDITION)

-   -   a. V₁=0V    -   b. V₂=−28.275 kV

VARIANT B EXAMPLE 1

-   -   a. V₁=+75V    -   b. V₂=−21.3 kV

VARIANT B EXAMPLE 2 (TOP FIELD PLATE GROUNDED CONDITION)

-   -   a. V₁=0V    -   b. V₂=−21.375 kV

VARIANT C EXAMPLE 1

-   -   a. V₁=+75V    -   b. V₃=−75V, V₂=−14.1 kV

VARIANT C EXAMPLE 2

-   -   a. V₁=+75V    -   b. V₃=−226V, V₂=0V

Although some common-mode charging will occur and higher bottomelectrode voltage V₂ may be required when example 2 of variant A isused, this embodiment can be particularly useful by only requiring agrounded, nonpatterned field plate for most lateral LED structures.

In yet another embodiment, the top field plate can have a thickerdielectric and require a higher voltage V₁. This may have advantages inlessening top device structure topology effects, facilitate non-contacttesting and improve the robustness of the top field plate from slightdielectric imperfections such as scratches and the like. For example, a200 μm quartz field plate dielectric (element (f) in the previousexample) and a 10 um air gap would require for the variant Aconfiguration of FIG. 20 the following voltages:

-   -   a. V₁=+17.4 kV    -   b. V₂=−28.5 kV        These voltages were chosen to avoid net charge transfer to the        devices, essentially keeping the LED device structure close to        ground potential. It is important to recognize that an air gap        will breakdown and become ionized if the electric field strength        is high enough. According to Paschen's Law for air at standard        pressure and temperature, a 10 μm air gap will breakdown at        about 350V while the above example assumed over 2.8 kV could be        maintained across the air gap. Instead, for V₁ voltages        exceeding about +2.2 kV in this example, the air gap will become        ionized and lower the voltage drop across the air gap. The        required V₁ will therefore be closer to +14.5 kV assuming no        voltage drop across the ionized air gap. Of course, other        voltage waveforms and values are possible to achieve the desired        current injection conditions. Of course, there can be other        variations, modifications, and alternatives.

When using a patterned field plate electrode pattern such as variants Band C of FIG. 20, a specific electrode pattern would be required forevery lateral device structure and the top field plate dielectricthickness must be less than or of the same order as the device pitch toavoid excessive electric field cross-coupling. Moreover, accuratepositional registration of a patterned top field plate between the fieldplate 2002 and the support substrate is important to maximize couplingefficiency and minimize parasitic capacitances (for example betweenelectrode 2007 and the n-layer cathode contact area). Positionalregistration between the electrode and the LED structure within +/−5% ofthe device size is believed to be adequate to allow efficient couplingand reproducible C²I measurement. For multiple electrode structures,such as variant C of FIG. 20, registration is also important to avoiddamaging the devices under test. For example, mis-registering the fieldplate to the device structures could negate current injection or evendamage the LED devices by injecting substantial reverse bias voltages.

Certain image processing methods can be utilized to improve the accuracyof the measured data corresponding to each LED device under test. Eachimaged LED device onto the sensor would be imaged onto a specific areawithin the camera sensor array. One image processing method uses spatialinformation from the target image to generate a physical centroid (x,y)location for each LED device within the measured camera output dataimage. This correspondence of LED device centroid location on thesupport substrate to its corresponding centroid location on the camerasensor can be developed and possibly corrected using cameramagnification, optical distortion correction, image capture to sense andlocate the LED device matrix and the like. The resulting centroid matrixwould therefore be the set of (x,y) location within the sensor image foreach LED device. For example, referring to the previous example, a960×600 LED device set imaged onto a 1920×1200 digital sensor matrixwould have a centroid matrix as follows:

Centroid for LED (i,j)=Camera data location (x.y)

Where i, j are integers (i=1 to 960, j=1 to 600) for each measured LEDwhile the camera location (x,y) is a floating point number within thesensor pixel area (0<x<1920, 0<y<1200). Once this centroid matrix isdeveloped, image processing methods using weighted functions can takethe digitized image and develop a set of data values that are extractedusing a weighing function where more weight is given to sensor dataimaged closest to the physical LED centroid location. Image processingsystems can accomplish this convolution function in parallel and usuallyat frame rate speeds. The LED data values thus comprises of an outputLED device (i,j) matrix of data values calculated using centroidweighted functions applied to the digitized camera data in a preferredembodiment.

While the above describes the key steps to C²I data image capture toyield an output LED device (i,j) data point, offset andscaling/normalization operations can also be applied. For example, a“dark” image captured without the voltage waveforms would measure thedark signal for each camera pixel at the current image acquisitionparameters. These dark images can be acquired along with each C²I datacapture as a form of offset and drift cancellation. The reference can besubtracted from the image data or after both data and reference havebeen processed using centroid weighted functions described above toyield an offset-corrected LED device (i,j) data matrix. Scaling andnormalization operations are also possible.

Additional image processing methods applied to the digitized cameraoutput (proportional to the total integrated light emitted by the LEDdevices imaged onto the camera sensor(s) during phase I) can be utilizedto develop a result indicative of LED device functionality. Thisfunctional data will be in the form of a matrix that contains one ormore values derived from the measurement. For each LED at location(i,j), there will be a set of n data points Data_(n)(i,j)=Value_(n)(where n is an integer greater or equal to 1). Multiple independentData_(n)(i,j) values for each LED under test for example, could bevalues of light output at differing current density values measuredusing n measurement sequences taken with different phase I voltage rampvalues. Each Data_(n)(i,j) measurement data value can in turn be theaverage of multiple measurements to improve signal-to-noise ratio.Signal averaging is a well-known method where the standard deviation ofa signal exhibiting stochastic noise would be reduced by sqrt(m) where mis the number of measurements points that are averaged. For example, ifa data point exhibiting a stochastic noise standard deviation of z,averaged data points using the average of 100 data points would have astandard deviation of z/sqrt(100) or 10 times lower.

Once the LED device (i,j) data values are collected, a threshold or setof test criteria can be applied to develop a determination offunctionality, perhaps adding a Data_(n)(i,j) value of 0 or 1 (0=baddevice, 1=good device) for each LED being measured. For example,non-emitting or weakly emitting devices could be labeled as bad devicesif a desired minimum threshold is applied to the data. Of course,multiple thresholds and other criteria applied to the set of data valuesor the pass/fail criteria could also be useful in functional test,repair strategies and process yield analysis (cause and correction). Asmerely an example, multiple thresholds could be applied to the LEDdevice Data_(n)(i,j) data to generate a bin number label for each LEDdevice to match LEDs in functionality and drive a strategy of releasingdevices with similar characteristics according to a criteria or set ofcriteria. Random-access laser lift-off or other individual LED devicerelease methods could aggregate LED devices having similar bin numbersbased on the bin label matrix value for each (i,j) LED device. Thiscould be useful to limit display non-uniformity caused by using LEDdevices having excessively different functional characteristics.Multiple thresholds could also be utilized to develop statistics usefulfor yield and process control. For example, the standard deviation andother statistical analyses applied to bin data can be an indicator ofyield and process stability. Sudden changes in these derived quantitiescan signal a process excursion. FIG. 21 shows a histogram plot 2100 ofseveral LED devices falling within small ranges of Data_(n) values(called channels or bins) on the vertical scale as a function ofData_(n) in the horizontal scale. Most of the LED devices fall within afunctionally acceptable range 2101 while LED devices below threshold2102 or above threshold 2103 are considered rejects. The width 2104 ofthe LED device binning function can be useful for yield and processcontrol. LED devices falling within similar bins 2105 could be lateraggregated and used for their similar functional test results to improvedisplay uniformity.

If the functional test apparatus according to this invention images lessthan the desired area and requires a step and repeat function, thecentroid matrix may need to be recalculated for each new LED device areato be measured. If the step system is sufficiently accurate to align thenext set of LED devices to be measured however, the centroid matrix maybe reused. Of course, there can be other variations, modifications, andalternatives.

Generally, a field plate allows functional testing of a substratecontaining LED devices to occur either by one or more cameras fixed ormoved relative to the field plate. Test equipment cost, complexity,target LED device size and test throughput capability are some of thecriteria that must be evaluated before a specific configuration isselected. Other design limitations and criteria must also be addressedto assure measurement functionality to desired specifications. One suchdesign criteria is assuring the phase I voltage waveform across each LEDdevice being tested is not significantly distorted due to contactresistance and parasitic capacitance. For example, a fast voltage rampfor Phase I desired to measure higher current density operation couldcause a significant waveform distortion and voltage drop caused by RClow-pass filtering for LED devices situated in the middle of the fieldplate. This could occur if the field plate electrode or common contactresistance is too high. Mitigation of these effects could happen bylowering the effective contact sheet resistivity or attaching a lowerresistivity layer before testing. Finally, a large field plate willrequire power to charge & discharge the field plate capacitance C_(FP)at the measurement repetition rate and may generate resistive heatingwithin the contact layers. For example, a 6″ substrate field plate usinga 3 μm silicon dioxide dielectric layer would have a total capacitanceC_(FP) of about 200 nF. If a 16 Hz capture rate and 500V ramp isassumed, the ½ CV²f power would be about 0.5 W. At this proposedoperating point, small and manageable test power levels are generated,even with a full 6″ field plate configuration.

Application of the C²I method within a roll-roll film environment suchas an OLED manufacturing line may require additional elements such asmechanical stages and multiple sensor assemblies. For example, C²Itesting using field plates on each side of the film within a roll-rollcontinuous film environment could be realized by simply moving the testassembly at the same speed as the film. FIG. 22 shows a roll-roll withC²I test 2200 where C²I measurement assemblies 2201 and 2202 are movedusing motions 2203 and 2204 to follow the film towards the right tobecome stationary to the film in a relative sense during the test(measurement and discharge phases). The roll-roll manufacturing processis composed of a feed roll 2205 having a film 2206 supporting an LEDstructure 2207 to be tested. The film is moved to a take-up roll 2208 ata speed 2209. The figure shows a completely non-contact measurementconfiguration where top field plate 2210 and bottom field plate 2211 areplaced in close proximity to the film 2206. The top field plate 2210consists of an electrode 2212, dielectric 2213 and a gap 2214. Thebottom field plate 2211 consists of at least an electrode 2215 and anoptional dielectric 2216 and gap 2217 below the LED structure andsupport substrate/film. A camera or array of cameras 2218 and 2219complete the C²I system. As merely an example, if the total test time(measurement through discharge phase) is 60 msec and the film isadvancing at 1 m/s, the measurement field plate assembly would have tomove with the film for a distance of 60 mm before moving back to enableanother cycle. To avoid measurement gaps caused by the finite time tomove the measurement system back to its start position, a second C²Isystem 2202 is used to overlap and cover the test areas. The width ofthe field plate 2210 and 2211 should also be about 60 mm to minimizegaps and maximize overlaps between the C²I measurement systems. Ofcourse, there can be other variations, modifications, and alternativesto embodiments that move the field plates within a continuousmanufacturing process.

Embodiments where a C²I system is stationary within a roll-rollmanufacturing process can be desirable. FIG. 23 shows a similarroll-roll LED structure on a support film manufacturing process 2300with rolls 2301 and 2302 rotating to move film 2303 at a velocity 2304.A stationary C²I system 2305 comprising a camera 2306 and top and bottomfield plates 2307 and 2308 that are energized using measurement anddischarge phase waveforms according to this invention. In this example,the measurement phase is assumed to be short compared to film velocity2304 and thus limited blurring occurs during the measurement phase. Asan example, a 25 μsec measurement phase at a film transport velocity of500 mm/sec will only blur the image by 12.5 μm. This limited image smearwill be acceptable and unlikely to adversely affect the totalphoto-electron count measurement. Since the C²I system is stationary(and assuming the field plate width is the test amd discharge timemultiplied by the film velocity), the film just inside of the left sideof the field plates will have seen the full test and discharge cyclesince it will exit the right side just as the discharge cycle will haveended. For other film areas, however, the film will exit the field plateregion before the discharge phase is completed. This abrupt exit fromthe electric field region may damage the LED devices. A secondary platesystem is thus necessary to safely discharge every region tested by thestationary C²I system 2305. This additional discharge plate system 2309consisting of special field plates 2310 and 2311 resets the electricfield intensity of the LED device structure and film 2303 to a knownstate in order to safely discharge the film in a repeatable and uniformmanner. In the following examples, the initial dielectric thickness andother plate parameters are assumed to be equal to the C²I system fieldplates and operating bias conditions. Other combination of parameterssuch as voltage bias, plate dielectric thickness and interaction widthare possible to perform the discharge phase.

During the discharge state, a charge density Q′ (coulomb s/cm²) storedwithin the field plates, gaps, LED structure and support substrate mustbe discharged in a manner that limits the LED reverse bias levels tosafe values. A constant reverse bias leakage current density −I_(L)′ isassumed. FIG. 24A-B show two embodiments 2400 and 2401 of the dischargeplate system. As the film 2402 enters the right discharge plateassemblies at location 2403, the charge state is raised by voltagepotentials V₁ and V₂ applied to at least the leftmost region of thedischarge plate assembly. These voltages are selected to be the fullvoltages used during the measurement phase to the top and bottom platesrespectively. At that state, the charge density is related to voltage V₁& V₂ as follows:

Q′=C′ _(EFF) ×V   (17)

-   Q′=Total initial charge density in the discharge plate assembly    (coul/cm²)-   C′_(EFF)=Effective capacitance per unit area of the measurement    system (F/cm²)-   V=V₁+V₂, the maximum test voltages (V₁ is assumed to be opposite    polarity from V₂)    At a constant discharge rate (dQ′/dt=−I_(L)′):

−I _(L) ′=C′ _(EFF) ×dV/dt+V×dC′ _(EFF) /dt   (18)

If the time derivatives of equation 18 above are in turn related to thefilm velocity, a spatial change of dV/dt or dC′_(EFF)/dt over the widthof the discharge plate assembly satisfying equation 18 would safelydischarge the LED structure. FIG. 24A shows a safe discharge usinglinearly decreasing voltages from V₁ to 0 on the top field plate and V₂to 0 on the bottom field plate in the discharge plate assembly 2400. Thewidth of the discharge plate would be selected to satisfy equation 18 atthe film velocity.

If the dielectric thickness of the top and bottom discharge field platesare instead increased over the width of the discharge plate in a spatialmanner so as to satisfy the second term of equation 18, safe dischargeof the LED structure will also occur. FIG. 24B shows this secondembodiment using constant V₁ and V₂ over the width of the dischargeplate assembly 2401. Alternatively, a combination of embodimentsdescribed in FIGS. 24A and 24B could also be used to generate thedesired discharge phase. Of course, there can be other variations,modifications, and alternatives to embodiments that have stationaryfield plates within a continuous manufacturing process.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Although the above has been described using a selected sequence ofsteps, any combination of any elements of steps described as well asothers may be used. Additionally, certain steps may be combined and/oreliminated depending upon the embodiment. Furthermore, although thedescription and examples has been directed towards GaN LED devices on aplanar surface, any planar or curved surface containing photon emittingdevices could be functionally tested using the C²I method. For example,Vertical-Cavity Surface-Emitting Lasers (VCSELs), Organic LEDs (OLEDs),silicon photonics devices and other surface emitting devices could betested using this invention. Additionally, in another example, II-VIsemiconductor materials and associated devices can also be used. In anexample, the LED or other device can have a variety of applications suchas general or specialized lighting, a display, whether a large panel,mobile device, or projection, chemical treatment, vehicle lighting,medical, and others. In an example, the method can also includeselecting one of the good devices, releasing said LED device onto amember substrate and packaging the device. A member substrate caninclude the final product substrate or a temporary substrate to receivethe released LED device. The package can be a standard can, chip onboard, or submount, or module device. After the device is packaged, itcan be configured in one of a variety of applications. Of course, therecan be other variations, modifications, and alternatives. Therefore, theabove description and illustrations should not be taken as limiting thescope of the present invention which is defined by the appended claims.

What is claimed is:
 1. An apparatus for observing light emission from alight-emitting device structure disposed on a support substrate having afirst contact layer accessible from a surface and a second contact layerincluded on the light-emitting device structure, the light emittingdevice structure being selected from either a vertical light emittingdevice structure or a lateral light emitting device structure, theapparatus comprising: a field plate device, the field plate devicehaving a first face and a second face opposing the first face, thesecond face comprising a conductive layer, and an overlying dielectriclayer, the dielectric layer being positioned in close proximity to atleast a portion of the first contact layer of the light-emitting devicestructure; a voltage source for producing a voltage, the voltage sourcebeing capable of generating a time-varying voltage waveform, the voltagesource having a first terminal and a second terminal, the first terminalhaving a first potential coupled to the conductive layer of the fieldplate device, the second terminal at a second potential, the voltagesource being capable of injecting a capacitively coupled current to thelight emitting device structure to cause at least a portion of thelight-emitting device structure to emit electromagnetic radiation in apattern; and a detector device coupled to the light emitting devicestructure to form an image of the electromagnetic radiation in thepattern derived from the light-emitting device structure.
 2. Theapparatus of claim 1 wherein the second terminal is electrically coupledto the backside of the support substrate, the second potential being atground potential or at negative potential or positive potential inrelation to the ground potential to create the time-varying voltagewaveform.
 3. The apparatus of claim 1 wherein the first terminal iselectrically connected to the conductive layer of the field plate deviceand being at a ground potential or at a negative potential or a positivepotential in relation to the ground potential to create the firstpotential.
 4. The apparatus of claim 1 wherein the second face of thefield plate device comprising the dielectric layer is positioned on andin contact with the portion of the first contact layer.
 5. The apparatusof claim 1 wherein the second face of the field plate device comprisingthe dielectric layer is positioned in close proximity to form a spatialgap between the second face and the portion of the first contact layer.6. The apparatus according to claim 1 wherein the lateral light emittingdevice structure comprises a first contact layer and a second contactlayer, the first contact layer and the second contact layer beingelectrically accessible on at least one face of the lateral lightemitting device structure.
 7. The apparatus according to claim 1 whereinthe image is a derived from a light output of an emitting surface of thelight emitting diode structure resulting from application of acapacitively coupled time varying voltage waveform.
 8. The apparatusaccording to claim 3 wherein the conductive layer is patterned andcomprises a first portion within a vicinity of the first contact layerand a second portion within a vicinity of the second contact layer, thefirst portion being electrically and physically separated from thesecond portion the first portion being connected to the first terminalof the voltage source, and the second portion being connected to anothervoltage source or a ground potential.
 9. The apparatus according toclaim 3 wherein the conductive layer comprises a first portion within avicinity of the first contact layer and an absence of a conductive layerwithin a vicinity of the second contact layer.
 10. The apparatusaccording to claim 1 wherein the vertical light emitting devicestructure comprises the first contact layer and the second contact layerunderlying the light emitting device structure.
 11. The apparatusaccording to claim 1 further comprising a lens coupled to the detectordevice for focusing the electromagnetic radiation provided on thedetector device.
 12. The apparatus according to claim 1 wherein thedetector device comprises imaging the electromagnetic radiation toproduce an observable map of the pattern of electromagnetic radiation asa function of position over the light-emitting device structure of thesupport substrate.
 13. The apparatus according to claim 1 whereindetector device comprises a camera; and further comprising an electricalaccess coupled to the second contact layer of the light-emitting devicestructure using an electrical contact or using capacitive coupling. 14.The apparatus according to claim 1 wherein the time-varying voltagewaveform is a voltage ramp from a first voltage potential to a secondvoltage potential to forward bias the light-emitting device structure atselected current density during the measurement phase.
 15. The apparatusaccording to claim 13 wherein the camera integrates the electromagneticradiation over the time-varying voltage waveform to produce a spatialmap of total electromagnetic radiation produced over the light-emittingdevice structure.
 16. The apparatus according to claim 15 wherein thespatial map of integrated electromagnetic radiation is processed usingimage processing device to perform one or more of the followingfunctions: signal averaging, thresholding and binning to develop aspatially-dependent functional test result of the light-emitting devicestructure.
 17. The apparatus according to claim 1 wherein the firstcontact layer of the light-emitting device structure is isolated using amaterial removal process to realize a plurality of individuallyaddressable light-emitting devices.
 18. The apparatus according to claim1 wherein the first and second contact layers of the light-emittingdevice structure are isolated using a material removal process torealize a plurality of individually addressable light-emitting devices.19. The apparatus according to claim 14 wherein the time-varying voltagewaveform after the measurement phase is returned from the second voltagepotential to the first voltage potential over a time period called thereset phase selected to use the light-emitting device reverse biasleakage current density and avoid exceeding potentially damaging reversebias voltage.
 20. The apparatus according to claim 13 wherein the camerais one of a plurality of cameras, each positioned to image a separatearea of the light-emitting device structure.
 21. The apparatus accordingto claim 13 wherein the camera and a smaller field plate device is anassembly that can image a smaller test area and mechanically indexed ina step and repeat fashion for more complete test coverage.
 22. Theapparatus according to claim 1 wherein the field plate device isapproximately the same areal dimension to the support substrate and isplaced on the support substrate to allow substantially completefunctional testing of the support substrate without step and repeatindexing of the field plate device.
 23. The apparatus according to claim1 wherein the field plate is placed in close proximity to the supportsubstrate using a seal near the periphery of the field plate and air isevacuated from the gap using a vacuum port.
 24. The apparatus accordingto claim 1 wherein the close proximity between the field plate and thesupport substrate device is actual contact.
 25. The apparatus accordingto claim 1 wherein the close proximity between the field plate deviceand the support substrate device is characterized by a small gapcomprising one or more of a gas, a vacuum, a liquid or a solid, thesmall gap being no larger than 5 times the lateral distance of an LEDdevice to be formed from the light emitting diode structure.
 26. Amethod of manufacturing an optical device, the method comprising:providing a light emitting diode structure, the light emitting diodestructure having a plurality of LED devices to be formed, disposed on asupport substrate having a first contact layer accessible from a surfaceand a second contact layer provided on the light-emitting devicestructure, the light emitting diode structure being either a verticallight emitting diode structure or a lateral light emitting diodestructure; coupling a field plate device to the light emitting diodestructure, the field plate device having a first face and a second faceopposing the first face, the second face comprising a conductive layer,and an overlying dielectric layer, the dielectric layer being positionedin close proximity to at least a portion of the first contact layer ofthe light-emitting device structure such that a spatial gap is formedbetween a surface region of the dielectric layer and the first facecontact layer of the light emitting device structure; generating atime-varying voltage waveform from a voltage source to form a voltagepotential between the dielectric layer of the field plate device and thelight emitting device structure to inject current to at least aplurality of the LED devices in the light emitting device structure tocause the light-emitting device structure to emit electromagneticradiation in a pattern; and capturing, using a detector device coupledto the light emitting device structure, an image of the electromagneticradiation in the pattern derived from the light-emitting devicestructure.
 27. The method according to claim 26 wherein the detectordevice comprises imaging the electromagnetic radiation to produce anobservable map of the pattern of electromagnetic radiation as a functionof position over the light-emitting device structure of the supportsubstrate.
 28. The method according to claim 27 wherein detector devicecomprises a camera.
 29. The method according to claim 28 wherein thefield plate device is transmissive and the camera is mounted to imagethe light-emitting device structure to collect the electromagneticradiation through the field plate device.
 30. The method according toclaim 28 wherein the support substrate is transmissive and the camera ismounted to image the light-emitting device structure to collect theelectromagnetic radiation through the support substrate.
 31. The methodaccording to claim 26 wherein the time-varying voltage waveform is avoltage ramp from a first voltage potential to a second voltagepotential to forward bias the light-emitting device structure atselected current density during the measurement phase.
 32. The apparatusaccording to claim 28 wherein the camera integrates the electromagneticradiation over the time-varying voltage waveform to produce a spatialmap of total electromagnetic radiation produced over the light-emittingdevice structure.
 33. The apparatus according to claim 32 wherein thespatial map of integrated electromagnetic radiation is processed usingimage processing device to perform one or more of the followingfunctions: signal averaging, thresholding and binning to develop aspatially-dependent functional test result of the light-emitting devicestructure.
 34. The method according to claim 26 wherein the time-varyingvoltage waveform after the measurement phase is returned from the secondvoltage potential to the first voltage potential selected to use thelight-emitting device reverse bias leakage current density and avoidexceeding potentially damaging reverse bias voltage.
 35. The methodaccording to claim 26 wherein the first contact layer of thelight-emitting device structure is isolated using a material removalprocess to realize a plurality of individually addressablelight-emitting devices.
 36. The method of claim 26 wherein the first andsecond contact layers of the light-emitting device structure areisolated using a material removal process to realize a plurality ofindividually addressable light-emitting devices.
 37. A method ofmanufacturing an optical device, the method comprising: providing alight emitting diode structure, the light emitting diode structuredisposed on a support substrate having a first face contact layeraccessible from a surface and a second contact layer underlying thelight-emitting device structure; coupling a field plate device to thelight emitting diode structure, the field plate device having a firstface and a second face opposing the first face, the second facecomprising a conductive layer, and an overlying dielectric layer, thedielectric layer being positioned in close proximity to at least aportion of the first contact layer of the light-emitting devicestructure; generating a time-varying voltage waveform from a voltagesource to form a voltage potential between the dielectric layer of thefield plate device and the light emitting device structure to injectcurrent to a plurality of the LED devices in the light emitting devicestructure to cause the light-emitting device structure to emitelectromagnetic radiation in a pattern; and capturing, using a detectordevice coupled to the light emitting device structure, an image of theelectromagnetic radiation in the pattern derived from the light-emittingdevice structure.
 38. The method according to claim 37 wherein thedetector device comprises imaging the electromagnetic radiation toproduce an observable map of the pattern of electromagnetic radiation asa function of position over the light-emitting device structure of thesupport substrate.
 39. The method according to claim 37 wherein thetime-varying voltage waveform is a voltage ramp from a first voltagepotential to a second voltage potential to forward bias thelight-emitting device structure at a selected current density during themeasurement phase.
 40. The method according to claim 37 wherein thefirst contact layer of the light-emitting device structure is isolatedusing a material removal process to realize a plurality of individuallyaddressable light-emitting devices.
 41. The method according to claim 37wherein the first and second contact layers of the light-emitting devicestructure are isolated using a material removal process to realize aplurality of individually addressable light-emitting devices.
 42. Themethod according to claim 37 further comprising selecting at least oneof the LED devices, and packaging the LED device.
 43. The methodaccording to claim 37 further comprising selecting at least one of theLED devices, releasing said LED device onto a member substrate.
 44. Amethod of manufacturing an optical device, the method comprising:providing a light emitting diode structure overlying a substrate member,the light emitting diode structure disposed on a support substratehaving a first face contact layer accessible from a surface and a secondcontact layer underlying the light-emitting device structure; coupling afield plate device to the light emitting diode structure, the fieldplate device having a first face and a second face opposing the firstface, the second face comprising a conductive layer, and an overlyingdielectric layer, the dielectric layer being positioned in closeproximity to at least a portion of the first contact layer of thelight-emitting device structure; generating a time-varying voltagewaveform from a voltage source to form a voltage potential between thedielectric layer of the field plate device and the light emitting devicestructure to inject current to a plurality of the LED devices in thelight emitting device structure to cause the light-emitting devicestructure to emit electromagnetic radiation in a pattern; and capturing,using a detector device coupled to the light emitting device structure,an image of the electromagnetic radiation in the pattern derived fromthe light-emitting device structure; processing light emitting devicestructure overlying the substrate to either singulate the light emittingdevice structure or perform another process on the light emitting devicestructure; coupling at least a pair of interconnect members to the lightemitting device structure; and integrating the light emitting devicestructure into an application, the application being selected from ageneral lighting device, a luminaire, a display, a projector, a lamp fora vehicle, or a beam light, or a specialty lighting device.
 45. Themethod of claim 44 wherein the LED devices in the light emitting devicestructure are free from a probe mark or other test mark.